regular-language

问题 How to calculate minimum pumping length of a regular language. For example if i have 0001* then minimum pumping length for this should be 4 ,that is 000 could not be pumped . Why it is so? 回答1: It will be less than or equal to the number of states in a minimal DFA for the language, minus one. So convert the regular expression into a DFA, minimize it, and count the states. For your example: 0001* SOURCE SYMBOL DESTINATION q1 0 q2 q1 1 q5 q2 0 q3 q2 1 q5 q3 0 q4 q3 1 q5 q4 0 q5 q4 1 q4 q5 0 q5

问题 I don't know how to do this, and I've found no good resources online for how to perform this operation[.] I'm trying to take an annotated EBNF production rule which is a difference between two regular expressions and turn it into a(n a| f?)lex grammar specification rule[.] The problem is that I see no way to do this normally[.]{3} is there a way to do this using Kleene algebra, like the way you can use an empty match with alternation in a context-free grammar[?] 回答1: What does the EBNF

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 6 years ago . How can I show that the following language is (not) context-free? The argument that it's not regular goes as follows. I suspect this language to be context-free... The reason why I think this, is because L = {a n b m c {n+m} | n,m >= 0} is context-free. A proof for this can be found at http://cg.scs.carleton.ca/

问题 I have a single text file with 40,000 records. I need to locate all items greater than October 1st 2011. The format is 01-10-2011 - How can I do this using regular expression? 回答1: It probably shouldn't be done, but it can be done: ([0-3][2-9]|[1-3]1)-10-2011|[0-3][0-9]-1[12]-2011|[0-3][0-9]-[01][0-9]-201[2-9] This assumes all dates are DD-MM-YYYY and valid, and that you don't need to find dates further in the future than 2019, for which it could be adapted if necessary. Tested in Dreamweaver

问题 Let L(R) be the language denoted by regular expression R. I'd really love your help with presenting a regular expression to the complement of L((0 U 10 U 110)* (epsilon U 1 U 11)), where language is over the alphabet {0,1}, epsilon is the empty word, 'U' stands for union and '*' is the star iterator. I tried to find it with De Morgan's laws. I think that I am requested to evaluate not (L((0 U 10 U 110)* (epsilon U 1 U 11)))- what is not of the '*' for example? Thanks a lot 回答1: You need to

问题 I have an example string 1836248_NNY_01.pdf but it can also be 18362481_YYN_102.pdf And I need to get the nth character between the two underscores. So far my regex is \_(.*?)\_ to get the characters between the underscores. But following up I can't figure out how to get the 2nd N for example. https://regex101.com/r/XUMKyf/1/ 回答1: You could use \_.{1}(.).*\_ and replace the 1 with whatever you want. So 0 would be the first char, 1 the second, and so on. Example: https://regex101.com/r/XUMKyf

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 6 years ago . I did some googling on this and nothing really definitive popped up. Let's say I have two languages A and B. A = { w is a subset of {a,b,c}* such that the second to the last character of w is b } B = { w is a subset of {b,d}* such that the last character is b } How would one define this? I think the alphabet

问题 Specifically, I noticed that the language of regular expressions itself isn't regular. So, I can't use a regular expression to parse a given regular expression. I need to use a parser since the language of the regular expression itself is context free. Is there any way regular expressions can be represented in a way that the resulting string can be parsed using a regular expression? Note: My question isn't about whether there is a regexp to match the current syntax of regexes, but whether

问题 For the above automaton, the regular expression that has been given in my textbook is the following : a*(a*ba*ba*ba*)*(a+a*ba*ba*ba*) I am having trouble deriving this...the following is my attempt at it : aa* + aa*(ba*ba*ba*)* + ba*ba*ba* + ba*ba*ba*(ba*ba*ba*)* Either I am wrong or I am not being able to simplify it into the form given in the book. Can someone please guide me here, point out the mistake or explain it to me step-by-step? I'd really really thankful and appreciate that. 回答1:

问题 Given the alphabet {a, b} we define N a (w) as the number of occurrences of a in the word w and similarly for N b (w) . Show that the following set over {a, b} is regular. A = {xy | N a (x) = N b (y)} I'm having a hard time figuring out where to start solving this problem. Any information would be greatly appreciated. 回答1: Yes it is regular language! Any string consists if a and b belongs the language A = {xy | N a (x) = N b (y)} . Example: Suppose string is: w = aaaab we can divide this